Fig. 250.—Schema of primitive portal circulation.
3. Formation of Portal Circulation. A.—With the further development of the liver the direct connection of the distal segment of the vitelline veins with the sinus venosus becomes lost, the intermediate segment being entirely broken up into an intrahepatic network (Fig. 250). Hence all the blood brought to the liver by the vitelline veins (venæ hepaticæ advehentes) passes through the hepatic capillary circulation, before it is carried by the proximal segment of the vitelline veins (venæ hepaticæ revehentes) into the sinus venosus. The amount of this blood increases with new connections which the vitelline veins make with the venous radicles developing in the intestinal tract and its appendages. In proportion as, with the development of the placenta and reduction of the yolk-sac, the original significance of the vitelline veins as nutritive and respiratory vessels disappears, this secondary connection of the vitelline veins with the veins of the alimentary tract becomes more and more important, until finally the original vitelline veins, now properly called omphalo-mesenteric veins, return the blood from the intestinal tube, pancreas and spleen to the liver.
The venæ hepaticæ advehentes, becoming connected in this way with the developing intestine, pancreas and spleen, form the rudiments of the future portal system, while the venæ hepaticæ revehentes are prototypes of the hepatic veins of the adult circulation.
B. Development of the Portal Vein.—The distal subintestinal segments of the vitelline veins are early united by a transverse anastomotic branch. The section of the veins above this anastomosis is seen already in Fig. 250 to have assumed an annular shape, while the veins below the primary anastomosis are approaching each other to form a second ring-like junction.
In Fig. 251 the subintestinal segments of the two vitelline veins are seen to have communicated with each other by transverse anastomotic branches around the duodenum, two of these branches being situated ventrad and one dorsad of the intestinal tube. These branches, and the portions of the primitive vitelline veins between their points of derivation, form two vascular loops or rings, encircling the primitive duodenum (Fig. 251).
The distal portions of the vitelline veins, before reaching the caudal annular duodenal anastomosis, next fuse into a single longitudinal vessel which also receives the veins from the stomach, intestine, spleen, and pancreas, and forms the beginning of the portal vein.
By atrophy of the right half of the lower, and of the left half of the upper duodenal venous ring (Figs. 252 and [253]), the proximal portion of the portal vein is formed as a single vessel, taking a spiral course around the duodenum ([Fig. 256]). Hence in the adult the portal vein and its principal branch (the superior mesenteric vein) crosses over the ventral surface of the duodenum (third portion), turns along the mesal side of the second portion, and then continues to the liver along the dorsal aspect of the first portion ([Fig. 254]). Note—In comparing [Fig. 254] with the schematic figures it should be noted that the same presents the parts in the ventral view, while the schemata offer the dorsal aspect.
4. Changes Leading to the Final Arrangement of the Umbilical Veins.—A very important rearrangement of the umbilical veins takes place. These veins originally course in the lateral abdominal wall, close to the fold of the amnion ([Fig. 255]), and then turn cephalad of the developing liver along the septum transversum to empty into the sinus venosus at each end ([Figs. 249] and [250]). The right umbilical vein is at first the larger.